It is common practice to use a fusion seal as a means of joining component parts to form a composite article. Heretofore, fusion seals have been used in producing such articles as electric lamps, cathode ray tubes, and other display devices.
A major consideration in producing these articles has been a coefficient of thermal expansion (CTE) match. This has required that the ultimate seal have a CTE that is a reasonably close match with the CTEs of the component parts being sealed. In a cathode ray tube, for example, it is customary for the glass components to have a CTE on the order of 95-105.times.10.sup.-7 /.degree. C.
The present invention is concerned with optical articles, or devices, such as planar waveguides, waveguide gratings, couplers, and filters. In such articles, an optical fiber may be attached to a substrate having a near-zero, or a relatively large negative, CTE. For this purpose, a fusion seal must be firmly adherent to both the substrate and the fiber. The adherence must be sufficient to permit transfer of strain across the frit seal, that is, between the substrate and the fiber.
Index of refraction changes may be induced in optical fibers, such as silica and germania-silica fibers, by UV light. Fibers, so altered, are useful in producing complex, narrow-band optical components, such as filters and channel add/drop devices. These devices can be an important part of multiple-wavelength telecommunication systems. A reflective grating (or Bragg grating) is a photosensitive device which reflects light over a narrow wavelength band. Typically, these devices have channel spacings measured in nanometers.
Various constructions of optical filters are known which utilize the Bragg effect for wavelength selective filtering. One method for constructing a filter involves imprinting at least one periodic grating in the core of the optical fiber. The core is exposed through the cladding to the interference pattern of two ultraviolet beams. This results in a reflective grating which may be oriented normal to the fiber axis.
In silica and germania-silica fiber reflective gratings, variations in center wavelength are dominated by the change of refractive index with temperature. The frequency of the light reflected by the fiber grating varies with the temperature of the grating region. Consequently, such a filter cannot be used in applications where the reflected light frequency is to be independent of temperature.
The desirability of devising a system insensitive to temperature change is apparent. Provisional application Ser. No. 60/010,058, filed Jan. 16, 1996, describes an athermal device in which a thermally sensitive component having a positive CTE is affixed at two spaced locations on the upper surface of a substrate having a negative CTE. A lithia aluminosilicate glass-ceramic, beta-eucryptite, is suggested as a substrate to use in such a device. It is also taught that the item attached to the substrate, such as an optical fiber, may be attached by an organic polymer cement, an inorganic frit, or a metal.
It is a purpose of the present invention to provide an optical device comprising an optical component having a positive CTE in combination with a substrate having a near-zero, or negative, CTE. A further purpose is to provide such an article in which the component is attached to the substrate by a fusion seal. Another purpose is to provide a sealing material that has good sealing properties, that has a low CTE, and that will form an adherent seal between the optical component and the substrate. A still further purpose is to provide a method of producing such an optical device embodying a fusion seal.